The present invention relates to an improved pump unit, particularly for use in a dishwasher, and of the type including a pair of pumps which are operable by a common drive shaft.
A pump unit of this type is known, for example as disclosed in British patent specification No. 1,257,101, wherein the impellers of two separate pumps are keyed to a common drive shaft which is rotatable in opposite directions, such that one of the impellers rotates in a reverse direction when the other impeller is driven in a forward direction, and vice versa. Particularly, one pump is a centrifugal pump of the radial type having backward-oriented blades, and the other pump is of the so-called peripheral outflow or axial-flow type. When employed in a dishwasher, the centrifugal pump circulates water within a wash tub of the dishwasher, and the peripheral outflow pump discharges water from the wash tub.
In this known type of pump unit however, although the pump unit is of advantageously integral construction, it suffers from the drawback that during the water discharge phase, i.e. when the centrifugal pump is rotating in a reverse, non-operating direction, such centrifugal pump consumes a very high level of energy which is substantially wasted. Particularly, the energy consumption of a centrifugal pump of the radial type having backward-oriented blades is higher when its impeller rotates in the reverse direction than when it rotates in the operative, forward direction. It has been experimentally found that during the water discharge phase of the pump unit, the energy consumption of the centrifugal pump will amount to approximately 80% of the overall pump unit energy consumption. As a consequence, the electric drive motor of the pump unit must be undesirably oversized compared with the actual operating, i.e. pumping, requirements. Furthermore, the circulating pump, during the phase in which its impeller rotates in the reverse, nonpumping direction, is a source of hydraulic noise and vibration which should be reduced to a minimum.
It has been suggested, for example as disclosed in Italian No. 860,156, to provide a side wall of the pressure chamber of the centrifugal pump with a curved baffle extending inwardly. The baffle, or similar element, provides that, when the impeller of the centrifugal pump rotates in the reverse direction, a water ring which is formed in the pressure chamber is diverted so as to partially cross the suction inlet of the pump. As a consequence, possible incoming water is ejected through the suction mouth of the pump, whereas the water column at the delivery side reaches only a low level. It has been experienced however that such solution only slightly reduces the above discussed disadvantages. This primarily is due to a braking action which is produced on the impeller of the centrifugal pump by the water ring in the pressure chamber. On the other hand, this solution results in the centrifugal pump being of relatively complicated construction, and additionally provides a relatively limited yield during the regular pumping phases.